The present invention relates to radiation-curable organosilicon compositions; and in particular to organosilicon compositions which cure when exposed to ultraviolet light, herein also referred to as UV. More specifically the present invention relates to improved compositions which comprise an organosilicon compound bearing one or more (meth)acrylamide-substituted hydrocarbon radicals and an organosilicon compound bearing one or more mercapto-substituted hydrocarbon radicals.
Berger et al., U.S. Pat. No. 3,726,710, disclose silicone paper release compositions which consist of a vinyl-containing siloxane and a photosensitizer and which cure when exposed to high intensity, predominantly continuum, light radiation. Although the compositions of Berger et al. can contain various olefin-containing siloxanes and various substituted organic radicals, the specific disclosure of compositions containing (meth)acrylamide-containing siloxanes, with or without mercapto-containing siloxanes, is not approached.
The use of mercapto-containing silicon compounds in vinyl-containing silicone compositions has been disclosed by Gant, Canadian Pat. No. 1,029,491 and U.K. Pat. No. 1,433,461; by Bokermann et al., U.S. Pat. No. 4,052,529; and by Colquhoun et al., U.S. Pat. No. 4,070,526. In particular, Gant teaches that the use of a mercapto-functional silane or siloxane accelerates the cure of a composition containing a vinyl-containing siloxane and a siloxane bearing silicon-bonded hydrogen atoms. However, the use of mercapto-containing silicon compounds in UV-curable compositions which contain (meth)acrylamide-containing silicon compounds is not suggested by these references.
Pigeon, U.S. Pat. No. 4,290,869, teaches that the presence of --SH radicals, contained in certain mercaptoorganosilanes or siloxanes, contributes to a reduction in the amount of UV exposure needed to cure a photopolymerizable composition containing certain (meth)acryloxy-containing silanes or siloxanes. The silanes and siloxanes of Pigeon are prepared from alkoxysilanes and hydroxyl-terminated polydiorganosiloxanes in such a manner that they, and the photopolymerizable compositions prepared therefrom, contain not only (meth)acryloxy curing sites but moisture-reactive sites as well. Pigeon says nothing about (meth)acrylamide-containing siloxanes.
Preiner et al., European patent application No. 174 647 disclose UV-curable compositions which contain an acryloxy-terminated siloxane polymer, a mercaptoalkyl-substituted siloxane polymer, a photosensitizer and, optionally, an organotitanium or organotin compound. These compositions are said to be useful as potting compositions for electronic units. (Meth)acrylamide-containing compositions are not suggested by this reference.
Takamizawa et al., Japanese Publication No. 57-52371 (82-52371), disclose photosetting silicone compositions which contain a photosensitizer and certain (meth)acrylamide-containing siloxanes. Patentees' compositions are said to not suffer from the drawbacks that are exhibited by the (meth)acryloxy-containing siloxane compositions of the art, such as having a tacky surface after being cured by UV in an oxygen-containing atmosphere. However, patentees' compositions are limited to siloxanes which bear mono-(meth)acrylamide-substituted radicals so the oxygen inhibition of organosilicon compositions containing di-(meth)acrylamide-substituted radicals was not appreciated.
Varaprath, U.S. Pat. No. 4,608,270, discloses curable compositions which comprise di-(meth)acrylamide-containing silicon compounds. While these compositions are readily curable when exposed to UV light in the absence of oxygen, particularly when they contain a photosensitizer, it has been found that certain of these compositions require undesirably large amounts of UV in order to be cured to the tack-free state, in thick or thin section, when said exposure is done in an oxygen-containing atmosphere. With thick sections, such as coatings having a thickness of a few mils, the sub-surface portion of the coating will cure but the surface remains tacky.
An oxygen-free atmosphere and/or increased doses of UV can be used during the cure of a curable composition; however, either option is of limited value in commerce because of the additional expense associated therewith. Additionally the heat associated with UV radiation becomes excessive for some applications when large doses are used.
There is thus a need for silicone compositions which cure rapidly in air when exposed to UV radiation. The present invention provides those compositions.